1. Field of the Invention
The present invention relates to gas sensors, particularly a gas sensor adapted to control pump current through an oxygen pump element so as to hold the output voltage of an oxygen concentration cell element at a constant value and measure the oxygen concentration in measured gas on the basis of the pump current. More particularly, the present invention relates to a method of stabilizing pump current in a gas sensor through control of oscillations of the pump current which result from pulsations of measured gas.
2. Description of the Related Art
One of gas sensors heretofore known as an oxygen sensor, includes an oxygen concentration cell element made up of a pair of porous electrodes and an oxygen ion conductive solid electrolyte base interposed between the electrodes, and an oxygen pump element, the oxygen concentration cell element and the oxygen pump element being laid one upon another.
Such an oxygen sensor has a diffusion or measurement gap between the above described two elements. The measurement gap is in communication with a measured gas side by way of a diffusion control layer. By controlling current through the oxygen concentration cell element so that the output voltage of the oxygen concentration cell element is maintained at a predetermined constant value, the oxygen concentration in the measurement gap is controlled to a constant value. In this instance, the pump current through the oxygen pump element is proportional to the oxygen concentration in the measured gas, so that measurement of the oxygen concentration can be attained on the basis of that current value.
Such an oxygen sensor is attached to an exhaust pipe of a vehicle for instance and used for measuring the oxygen concentration in the exhaust gas (i.e., air-fuel ratio). In the exhaust pipe, the exhaust gas pulsates at cyclic intervals at which the engine performs its exhaust stroke. When the exhaust gas pulsates, the absolute quantity of oxygen varies depending upon variations of the density of the exhaust gas. Thus, the pump current through the oxygen sensor is influenced by the exhaust gas pulsations and caused to oscillate in the same cycle as the exhaust gas pulsations.
In the meantime, in the pump current control, there is a time lag causing a variation of the pump current after the output voltage is detected by the oxygen concentration cell element. Thus, depending upon an oscillation cycle of the pump current (i.e., the pulsation cycle of the exhaust gas), there may occur a hunting in the control, thus causing the pump current to oscillate at a larger amplitude than that to otherwise result from the actual oscillations. When the pump current is in such an oscillating condition, there arises a problem that accurate detection of the oxygen concentration cannot be attained on the basis of the pump current.
Another problem is that if such hunting occurs at the time of fuel cut where the oxygen concentration in the exhaust gas is increased to cause relatively large current to flow through the oxygen pump element, excessively large current flows through the oxygen pump element to deteriorate the oxygen pump element.
It is accordingly an object of the present invention to provide a method of stabilizing pump current in a gas sensor through suppression of oscillations of the pump current which result from pulsations of measured gas.
It is another object of the present invention to provide an oxygen gas sensor which is obtained by the above method.
To achieve the foregoing objects, the present invention provides a method of stabilizing pump current in a gas sensor through suppression of oscillations of the pump current which result from pulsations of measured gas. The gas sensor includes an oxygen pump element and an oxygen concentration cell element, each of which has a pair of porous electrodes containing a catalyst for accelerating a reaction between a solid electrolyte and oxygen and an oxygen ion conductive solid electrolyte base interposed between the electrodes. The gas sensor further includes a measurement gap between the oxygen pump element and the oxygen concentration cell element and in communication with a measured gas side by way of a diffusion control layer, and is operative to control the pump current through the oxygen pump element so that an output voltage of the oxygen concentration cell element is constant in order that an oxygen concentration in the measurement gap is maintained constant and detect an oxygen concentration in the measured gas on the basis of the pump current. The method comprises adjusting an activity of the electrodes of the oxygen pump element so as to change a responsiveness in control of the pump current and thereby reduce the oscillations of the pump current.
The present invention also provides a gas sensor comprising an oxygen pump element and an oxygen concentration cell element, each of which has a pair of porous electrodes containing a catalyst for accelerating a reaction between a solid electrolyte and oxygen and an oxygen ion conductive solid electrolyte base interposed between the electrodes, the gas sensor further comprising a measurement gap between the oxygen pump element and the oxygen concentration cell element and in communication with a measured gas side by way of a diffusion control layer, and operative to control the pump current through the oxygen pump element so that an output voltage of the oxygen concentration cell element is constant in order that an oxygen concentration in the measurement gap is maintained constant and detect an oxygen concentration in the measured gas on the basis of the pump current, wherein an activity of the electrodes of the oxygen pump element is adjusted so as to change a responsiveness in control of the pump current and thereby reduce an amplitude of the oscillations of the pump current.